A version of this article appears in Aqua Magazine: Why Colored Plaster Turns Whitish


 Kim Skinner

Having a beautiful pool with a colored surface, especially one with a quartz or pebble plaster finish, is a popular choice among pool owners, and understandably so. The color adds ambiance to the setting and can make the water wonderfully attractive and inviting. That’s why pool owners are willing to pay extra to have that special color enhance their water and the entire backyard.

Because the interior pool surface is such a dominating visual element, it must be disappointing for everyone involved on those occasions when they watch their pigmented, colored pool become unsightly with whitish blotches, streaks, or small spots. Later on, it must also be disappointing, if not more so, to learn that the cause of the unsightly color problem was misdiagnosed, and worse, the remedial action taken to resolve that problem caused more serious damage in the long run.

Plaster colors can fade or turn whitish for a handful of preventable reasons. First off, calcium scale can be deposited onto pigmented plaster surfaces by out-of-balance pool water. As is widely known, calcium scale makes the surface rough and turns it white, and acid washing or polishing is often the remedy.

However, it is also common to have colored plaster (including quartz and pebble) surfaces lighten in color due to mistakes in plaster workmanship and material selection. Because changes in color resulting from these defects are also a lighter or whitish color, it is easy to mistakenly believe that the problem is calcium scaling, and that the pool water has been out-of-balance. Once you make that misdiagnosis, it is then easy to mistakenly assume an acid wash is the best remedy. But when there’s no scale there to remove, acid washing will only cause etching, it won’t fix the problem and it will shorten the life of the plaster.

Plaster Workmanship

Here’s the improper workmanship issue that is both common and regrettably overlooked. The whitening of colored plaster can be caused by soluble calcium ions in the plaster dissolving and fleeing the scene into the pool water due to poor plaster workmanship and which has nothing to do with water balance. When calcium dissolution occurs, it means the plaster surface is deteriorating, losing density, and becoming porous. Yet it usually remains smooth. And when porosity develops and increases (which can take several months), a whitening of colored/pigmented plaster results.

A porous surface appears lighter in color because of the way it reflects more light than a non-porous surface. According to Dr. Boyd Clark, senior materials specialist with Construction Technology Labs, it’s the same reason that a head of foam is lighter than the beer; the foam is lighter in color because it throws off more light: “The fact that we have a spot, or a lightened area is due to the fact you have an increased porosity,” he says.

It’s helpful to note that even common white pool plaster can develop white spots and streaks that are whiter than the surrounding plaster because of increased porosity in those areas. There is a direct connection in how white and pigmented plaster develops color-shading changes.

Scale V. Porosity

Calcium scaling, which is caused by out-of-balance water (an overly positive LSI value), usually deposits a uniform layer of calcium carbonate over the plaster finish and whitens the entire pool. Even the surface of pebbles (in pebble finishes) will be covered by calcium scale including with the cement surface that surrounds and binds the pebbles together. Calcium scale is generally rough and is generally easy to remove by hand sanding with sandpaper.   

On the other hand, if the whitish discoloration is smooth to the touch, manifests itself in streaks, blotches, or small spots, and if diluted acid or hand sanding does not easily and quickly remove the white discoloration, then one should realize that the problem is probably not calcium carbonate scaling, but a porous surface instead that is deteriorating and turning white.

Additionally with pebble finishes, a very close examination is needed to determine if the whitish discoloration is only within the cement portion of the finish that surrounds and binds the pebbles. Porosity (a loss of material) only occurs in the cement portion and not the pebbles or quartz aggregates themselves. 

What leads to a porous, yet smooth surface? As mentioned above, there are plastering mistakes that lead to two relatively soluble components being slowly dissolved away that creates a very porous surface. Those soluble components are calcium hydroxide (a by-product of the cement/water reaction) and calcium chloride (if added to the mix to accelerate the hardening process).

Some people falsely assume that calcium hydroxide and calcium chloride can only be dissolved by aggressive pool water (negative LSI). That is not true. In fact, both plaster elements can be dissolved by positive LSI water. It is not aggressive water that causes porosity and a whitening effect to develop. 

The Plastering Issues

Given the above, poor plaster quality should not be overlooked when color problems develop during the first year or two. The onBalance team began examining failed colored pools in 1998. Plaster samples were sent to professional cement labs to perform SEM analysis. The labs were instructed to look for aggressive water effects, but in these cases none were found. The reports have come back from the labs that implicated poor workmanship and materials as reasons for color loss and whitening. The following are plaster mistakes mentioned by the labs.

[] A high amount of water added to the plaster mix contributes to excessive shrinkage (cracking and excessive crazing) and porosity over time.

[] An excessive amount of added calcium chloride, a hardening accelerator (often referred to as just “calcium” or “chloride”), can cause rapid hardening and troweling difficulties, and contributes to shrinkage, porosity, and blotchiness.

[] Applying large quantities of water to the surface while troweling, which increases porosity, shrinkage, deterioration, and color (shading) differences, while overly late hard troweling causes graying (mottling) of white plaster or a darker color in pigmented plaster.

There’s no question that poor workmanship and materials can result in a gradually and increasingly porous cement/plaster surface, including excessive shrinkage cracks, which also turn noticeably white. Those defective plaster practices lead to excessive porous surfaces that allow water, whether it’s balanced or imbalanced, to penetrate and dissolve soluble plaster components from the surface. It is the increase in porosity that results in a whitening effect in those compromised areas.

Quality Colored Plaster

In the cement/concrete industry, it is commonly known that producing a hard and dense cementitious surface is a major requirement to help ensure vivid and more consistent colors that last many years. A quality cement product can stand up to harsh conditions, such as aggressive water (the same as rain, which is very aggressive, on driveways or sidewalks), and not develop whiteness or color fade or loss.

When a quality plaster finish is produced with good workmanship and materials, just like concrete, it helps prevent color pigment(s) and other soluble plaster material from dissolving out of the plaster.  Below are some good workmanship practices suggested by the Portland Cement Association to produce a hard, dense, and long-lasting cementitious surface.

[] Use low water content (water/cement ratio) for the plaster mix. Make the plaster mix as thick as reasonable for proper troweling. That helps to provide a denser (less porous), harder, and more durable plaster finish.

[] Calcium chloride, a hardening accelerator (often referred to as just “calcium” or “chloride”), should not be added to colored plaster mixes. Pigment manufacturers recommend not adding calcium chloride as it can cause blotchiness. Also, tests have shown that about one-fourth of the added calcium chloride dissolves out of plaster, thereby increasing porosity.

[] The application and troweling of the plaster surface should be well timed to compact the plaster material properly and effectively. Avoid overly late hard troweling and having to add lots of water to the surface to make it more pliable for troweling. Water troweling is a detrimental practice that severely weakens and creates undesirable surface porosity and shrinkage cracks.

Other Factors

There’s another reason for color loss: some plasterers or suppliers use organic pigments in their plaster mixes. Organic pigments can be bleached and turn white by the presence of chlorine or other oxidizers, and even by the sun. It is recognized that organic pigments are cheaper than inorganic pigments, but they generally cannot withstand a swimming pool environment for very long.

This bleaching action occurs even if the plaster workmanship is of high quality. Some plaster mixes contain two pigment colors. If one is organic and becomes bleached, that color will simply disappear leaving the other non-bleachable pigment visible as the sole color.

How to Best Remedy Porous Plaster

An acid treatment is often tried to restore the original color of the plaster. However, such acid treatments often etch the plaster and make the surface rough. That in turn, leads to balanced water penetrating an etched surface and dissolving more of the soluble calcium hydroxide material away over time which then increases porosity and surface deterioration. That reduces the life span of the finish, and the plaster color often fades away and turns whitish once again.

With white plaster, an etched surface will more easily and quickly become stained with trapped dirt and minerals such as copper and iron. 

A better alternative for removing a porous and whitened plaster surface would be to sand/polish the plaster. That process removes the porous material and restores a hard, dense, and smooth surface.  Unfortunately, when dealing with discolored pebble finishes, sanding and polishing is virtually impossible.